1. Field of the Invention
This invention concerns low alloy steels for use in pressure vessel and, more specifically, it relates to Cr-Mo low alloy steels which are excellent in hardenability, hot strength, toughness, weldability and hydrogen attack and embrittlement resistance, as well as have excellent toughness even after the use in the temper brittle temperature region and, accordingly, are suitable to pressure vessel such as coal liquefying apparatus used in a hydrogen atmosphere under high temperature and high pressure.
2. Description of the Prior Art
Cr-Mo steels have hitherto been employed generally for pressure vessel such as in petroleum refining facilities used in the hydrogen atmosphere under high temperature and high pressure. By the way, new energy sources have recently been looked for as the substitutes for petroleum and study and experiment have been made, for example, on coal liquefication. In the case of coal liquefication, however, since the reaction is taken place under high temperature and pressure as compared with the conventional petroleum refining, reaction vessels used therefor have to satisfy the requirement for higher creep strength. Further, as the pressure vessel have become larger in the scale and increased in the thickness from the economical point of view, they tend to reduce the cooling rate and increase the time for post weld heat treatment thus making it difficult to provide steel materials with great hot strength. In addition, inevitable increase has been imposed to the material cost, and production or transportation cost due to the increase in the weight of the steel materials. Further, since the operation condition in the coal liquefication, for example, that of the temperature which is higher than 450.degree. C. corresponds to so-called the temper brittle temperature region, the toughness of the steels is degraded during use.
In order to overcome the foregoing problems, there have been proposed low alloy steels for use in pressure vessel, for instance, in Japanese Patent Publication No. 57946/1982 (Kokai 57-57946), in which the sulfur content is decreased to improve the toughness and the silicon content is decreased to suppress the sensitivity to embrittlement in Cr-Mo steels and, further, vanadium and niobium contents are added to compensate the reduction in the hot strength caused by the decrease in the silicon content. However, even these proposed steels have no sufficient hot strength and creep strength.